1. Field Of The Invention
The present invention relates to apparatus for correcting distortion in images scanned by electro-optical reflectors and particularly to apparatus for correcting distortion in a pattern form by scanning light beams in two directions with mirrors which are rotated by galvanometers.
The invention is especially suitable for use in computer output microfilmers (COM) wherein successive lines of alphanumeric characters are scanned across a film to record a page of computer output data. The invention is generally applicable for use in mirror scanning apparatus wherein beams of light are reflected off rotating mirrors for providing images with a high degree of accuracy and resolution.
2. Description Of The Prior Art
In computer output microfilmers, computer data can be converted into light beams which are scanned on to a film for recording the alphanumeric characters which are represented by the computer data. It has been found desirable to use a mirror which is rotated by a galvanometer for scanning the light beams to write a line of characters on the film. Successive lines may be written by advancing the film. However, in the interest of increased writing speed and accuracy, it is desirable to use a second galvanometer rotated mirror to scan the beams in a direction perpendicular to the direction of the lines; thus permitting a complete page of computer output data to be written, say in one frame of the film.
Inasmuch as it is necessary to record pages of very small size in the course of microfilming operations, it is necessary to eliminate any distortion. When the mirrors are rotated, however, the distance between the film and the effective center of rotation of the beams changes as the beams are scanned across the successive lines. There is a conical planar intercept of the beam in the film plane that results in a hyperbolic scan path, rather than a linear path along each of the successive lines. The resulting distortion manifests itself as a bowing of the lines which may be more pronounced at the top or bottom of the page depending upon the initial angle of the mirror which scans in the page direction with respect to the source of the light beam and the film. This distortion may be termed "pin cushion distortion".
In certain cathode ray tube apparatus, pin cushion correction circuits have been provided for controlling deflection of the electron beam (see U.S. Pat. Nos. 3,482,255; 3,566,181; and 3,649,870). Scanning apparatus using electro-optical reflectors such as galvanometer mirrors for scanning light beams do not provide control signals which are directly related to the distortion and thus cathode ray tube correction circuits are not applicable for correcting distortion in images scanned by electro-optical reflectors. Other schemes for correcting distortion have involved the use of special correcting lenses (see U.S. Pat. No. 3,144,511), and diffracting apertures (see U.S. Pat. No. 3,438,050). It has also been suggested to change the repetition rate of frequency of the galvanometer drive waveforms (see U.S. Pat. No. 3,372,398). Such changes may compensate for variations in the rate at which the record medium is advanced, but will not correct for distortion of the type mentioned above which results when individual lines are successively scanned.